SQL Does it make sense to re-order database instead of using ORDER BY to increase performance? - sql

Question

I have a database with around 120.000 Entries and I need to do substring comparisons (where ... like 'test%') for an autocomplete function. The database won't change.
I have a column called "relevance" and for my searches I want them to be ordered by relevance DESC. I noticed, that as soon as I add the "ORDER BY relevance DESC" to my queries, the execution time increases by about 100% - since my queries already take around 100ms on average, this causes significant lag.
Does it make sense to re-order the whole database by relevance once so I can remove the ORDER BY? Can I be certain, that when searching through the table with SQL it will always go through the database in the order that I added the rows?
This is how my query looks like right now:
select *
from hao2_dict
where definitions like 'ba%'
or searchable_pinyin like 'ba%'
ORDER BY relevance DESC
LIMIT 100
UPDATE: For context, here is my DB structure:
And some time measurements:
Using an Index (relevance DESC) for the search term 'b%' gives me 50ms, which is faster than not using an Index. But the search term 'banana%' takes over 1700ms which is way slower than not using an Index. These are the results from 'explain':
b%:
0 Init 0 27 0 0
1 Noop 1 11 0 0
2 Integer 100 1 0 0
3 OpenRead 0 5 0 9 0
4 OpenRead 2 4223 0 k(2,-,) 0
5 Rewind 2 26 2 0 0
6 DeferredSeek 2 0 0 0
7 Column 0 6 4 0
8 Function 1 3 2 like(2) 0
9 If 2 13 0 0
10 Column 0 4 6 0
11 Function 1 5 2 like(2) 0
12 IfNot 2 25 1 0
13 IdxRowid 2 7 0 0
14 Column 0 1 8 0
15 Column 0 2 9 0
16 Column 0 3 10 0
17 Column 0 4 11 0
18 Column 0 5 12 0
19 Column 0 6 13 0
20 Column 0 7 14 0
21 Column 2 0 15 0
22 RealAffinity 15 0 0 0
23 ResultRow 7 9 0 0
24 DecrJumpZero 1 26 0 0
25 Next 2 6 0 1
26 Halt 0 0 0 0
27 Transaction 0 0 10 0 1
28 String8 0 3 0 b% 0
29 String8 0 5 0 b% 0
30 Goto 0 1 0 0
banana%:
0 Init 0 27 0 0
1 Noop 1 11 0 0
2 Integer 100 1 0 0
3 OpenRead 0 5 0 9 0
4 OpenRead 2 4223 0 k(2,-,) 0
5 Rewind 2 26 2 0 0
6 DeferredSeek 2 0 0 0
7 Column 0 6 4 0
8 Function 1 3 2 like(2) 0
9 If 2 13 0 0
10 Column 0 4 6 0
11 Function 1 5 2 like(2) 0
12 IfNot 2 25 1 0
13 IdxRowid 2 7 0 0
14 Column 0 1 8 0
15 Column 0 2 9 0
16 Column 0 3 10 0
17 Column 0 4 11 0
18 Column 0 5 12 0
19 Column 0 6 13 0
20 Column 0 7 14 0
21 Column 2 0 15 0
22 RealAffinity 15 0 0 0
23 ResultRow 7 9 0 0
24 DecrJumpZero 1 26 0 0
25 Next 2 6 0 1
26 Halt 0 0 0 0
27 Transaction 0 0 10 0 1
28 String8 0 3 0 banana% 0
29 String8 0 5 0 banana% 0
30 Goto 0 1 0 0

Answer 1

Can I be certain, that when searching through the table with SQL it will always go through the database in the order that I added the rows?
No. SQL results have no inherent order. They might come out in the order you inserted them, but there is no guarantee.
Instead, put an index on the column. Indexes keep their values in order.
However, this will only deal with the sorting. In the query above it still has to search the whole table for rows with matching definitions and searchable_pinyins. In general, SQL will only use one index per table at a time; usually trying to use two is inefficient. So you need one multi-column index to make this query not have to search the whole table and get the results in sorted order. Make sure relevance is first, you need to have the index columns in the same order as your order by.
(relevance, definitions, searchable_pinyins) will make that query use only the index for searching and sorting. Adding (relevance, searchable_pinyins) as well will handle searching by definitions, searchable_pinyins, or both.